The field of art to which this invention pertains is aryl cyanate esters, i.e., cyanic acid esters of polyhydric phenols.
Industry is constantly searching for lighter, stronger and more resistant materials to be used in place of the materials used today. Structural and electrical grade composites made from thermoset or thermoplastic resins and glass or carbon fibers have been and are being used successfully in many applications. However, improved flame retardant and thermally stable compositions are being sought for printed circuit board applications and for structural composites used in aircraft cabins, such as honeycomb panels, floor beams, seats and the like. Composites made with brominated epoxy resins are flame retardant but lack thermal stability. One deficiency of halogenated flame retardant systems is delamination or conductor disbondment in printed circuit boards subjected to field-soldering operations where solder gun contact can elevate board temperature to 350.degree. C. A second deficiency in the use of halogenated resin systems is the generation of toxic, halogenated degradation by-products during fire exposure.
The use of laminates made with polyimides, e.g., adducts of bismaleimides and methylene dianiline, overcomes the above-stated disadvantages of using halogenated epoxy resins. However, polyimides have problems associated with high moisture absorption, brittleness, toxicity of unreacted aromatic diamines and poor storage stability of the resin solutions.
Cyanate esters of polyhydric phenols are finding increasing uses in many structural and electrical applications. Composites based on cured cyanate esters have many excellent properties, such as low moisture absorption, good heat stability and high tensile and flexural strengths.
Cyanate esters made from the reaction of a cyanogen halide and a variety of polyhydric phenols are disclosed in U.S. Pat. Nos. 3,562,214 and No. 4,060,541. Prepolymers of dicyanate esters of dihydric phenols are disclosed in British Patent No. 1,305,762. In U.S. Pat. Nos. 4,223,073 and 4,254,012, prepregs, composites and laminates based on cyanate esters of polyhydric phenols are described.
In U.S. Pat. No. 4,430,485, flame retardant polycarbonate and copolyestercarbonate compositions are described. Such compositions are made by incorporating sulfur-containing diphenols in the polycarbonate structures.